S. A. Dovbysh, “The splitting of separatrices and the generation of isolated periodic solutions in Hamiltonian systems with one and a half degrees of freedom”, Russian Math. Surveys, 44:2 (1989), 281–282

E. S. Shalimova, “Steady and periodic modes in the problem of motion of a heavy material point on a rotating sphere (the viscous friction case)”, Moscow University Mechanics Bulletin, 69:4 (2014), 89–96